Withdrawing and dispensing precise volumes of liquid without contaminating the liquid is a large and important part of many clinical applications and laboratory tests. While attempts to address these applications and tests manually have failed to provide the needed accuracy and purity, a number of automated or semi-automated liquid-measuring systems are presently being used to gauge more precisely the small liquid volumes to be withdrawn and/or dispensed in such applications and tests.
One of the more popular types of liquid-measuring systems uses motor-controlled pipette-like probes to aspirate or dispense a desired amount of fluid from or into a vessel. The probes are movably mounted over the vessel and, using a precision-controlled motor, are lowered into the vessel until the tip of the probe reaches a desired level below or above the upper surface of the liquid, or meniscus. A desired amount of liquid is then withdrawn from or dispensed into the vessel. Such systems have been designed with the goals of: minimizing cross-contamination between the contents of different vessels, avoiding splashing the liquid during the aspiration (deposition) process, and minimizing the portion of the probes that must be washed.
For example, liquid-measuring systems have been developed which require a vessel to be filled to a predetermined level, so that the pipette-like probe can be lowered to a predetermined position with the expectation that the probe has been precisely positioned with respect to the predetermined level. Unfortunately, such systems are disadvantaged, because the step of filling the vessel to a predetermined level is laborious, seldom precise and, when there is a limited volume of liquid available, sometimes impossible.
More recently developed liquid-measuring systems of this type control the position of the probe tip without previously knowing the upper level of the liquid in the vessel. This is accomplished by searching for the upper level of the liquid in the vessel as the probe tip is lowered into the vessel. By measuring some electrical phenomena associated with a change in the capacitance between the probe and the liquid in the vessel as the tip of the probe approaches the liquid, the system can then position the probe tip without the disadvantageous step of precisely establishing liquid at a predetermined level in the vessel. These systems typically search for the upper level of the liquid by sensing when the capacitance between the probe and the liquid reflects a change in voltage level that is greater than a threshold reference level.
A problem with this technique is that is dependent upon the sensitivity of the system's electronics monitoring the change in voltage level. If this sensitivity is inferior or degrades over time, the sensed voltage level change will be skewed and the system will be unable to compare accurately the change in voltage level to the threshold reference level. Further, an inferior or degraded ability to sense the change in voltage level results in delayed detection of the liquid level, which can cause the system to lower the probe beyond the desired level. This can result in contamination, splashing, etc., as previously discussed.
Another problem with this technique is that the sensed change in voltage level may not reach the threshold reference level in applications using, for example, small liquid volumes or certain types of liquids. Systems using a fixed threshold level are incapable of dealing with these applications, and systems providing the ability to adjust the threshold level require a laborious calibration process.
A previously unaddressed problem, which is believed to have been discovered in connection with the present invention, concerns unexpected capacitance-level changes in systems using multiple pipette-type probes for handling multiple liquid vessels. In applications using multiple probes to test the various liquid vessels, a capacitance change between one probe and its associated liquid vessel can significantly affect the capacitance between another of the probes and its associated liquid vessel. If not avoided, this seriously undermines the integrity of the operation involving the affected liquid vessel.
Accordingly, there is a need for a liquid-level sensing system, capable of precisely measuring capacitance between a probe and liquid, that can be implemented without the aforementioned shortcomings.